1. Field of the Invention
The present invention relates to a convergence adjustment circuit to adjust the convergence in a projection type display apparatus and a projection type display apparatus such as a projection television set provided with the convergence adjustment circuit.
2. Description of Related Art
In a projection television set which enlarges and projects an image formed by three picture tubes (e.g., cathode ray tubes) for three primary colors, the picture tubes are generally directed toward the screen at different angles and therefore misalignment of the image of different colors occur on the screen. Superposition of the three primary colors, so-called convergence, is achieved by generating an analog signal for convergence adjustment having a waveform of sawtooth or parabola which is synchronized with a horizontal and vertical scanning, while regulating the shape and signal level of the waveform in a control circuit.
In a conventional convergence adjustment circuit in a projection television set, an analog signal for adjustment which is outputted from a convergence-adjusting analog signal generation circuit is connected to variable resistors and the convergence is controlled by adjusting the value of the variable resistors. The variable resistors are often used in common in projection type display apparatus of different size and/or model. So that they are manufactured with difference by adjusting the value of each variable resistor corresponding to the size and/or model of the television set. Repairing the projection television set often requires adjustment of the value of the variable resistors. While it is necessary to install the variable resistors in an easily accessible place in order to facilitate such adjustment works, a recent projection television set has many as 40 variable resistors at the least, which make the layout of components in the television set very difficult. Since analog adjustment is not necessary adequate in terms of accuracy.
To eliminate the variable resistors, various convergence adjustment circuits employing digital convergence circuits were proposed as disclosed in the Japanese Patent Application Laid-Open No. 56-169983 (1981), the Japanese Patent Application Laid-Open No. 5-14912 (1993), and so on. These digital convergence circuits are not commercially practical because the convergence adjustment signal generator circuit and the circuit to control the output thereof are all made of digital elements and therefore components such as an A/D converter, memory and so forth cost more to be manufactured.
With the background described above, a hybrid convergence adjustment circuit combining analog and digital circuits which is expected to become dominant in the future will be described below.
FIG. 1 is a block diagram illustrative of a conventional hybrid convergence adjustment circuit combing analog and digital circuits. In FIG. 1, numeral 1 denotes a microcomputer which supplies control signals to a display signal generator circuit 2, a signal processor circuit 3 and digital adjustment circuits 5, 5. According to the control signal supplied from the microcomputer 1, the display signal generator circuit 2 supplies a character signal for the display of the number of channels, volume, setting function, and so on, to the signal processor circuit 3. The signal processor circuit 3 combines the character signal with video signals supplied from the outside, and supplies them to the picture tubes R, G, B which serve as display elements. The picture tube G, among the picture tubes R, G, B, is arranged at the center of the optical path of the projection system, and the picture tubes R, B are arranged on both sides of the picture tube G being inclined at specified angles to the optical path. The picture tubes R, G, B have deflection yokes R1, G1, B1, respectively, and the picture tubes R, B have auxiliary deflection yokes R2, B2, respectively, for the adjustment of convergence.
An analog signal convergence-adjusting generator circuit 4 constantly generates analog signals for convergence adjustment upon supply of power to the apparatus. The waveform and voltage level of the convergence adjustment analog signal is controlled by the digital adjustment circuit 5 according to the control signal supplied from the microcomputer 1. The signal controlled by the digital adjustment circuit 5 is sent to an amplifier 6 to be amplified, then sent to the auxiliary deflection yokes R2, B2 of the picture tubes R, B, respectively. The block diagram illustrates a representative constitution of a projection television set, and the circuits for the signals sent to the auxiliary deflection yokes R2 and B2 have an equivalent constitution.
In the convergence adjustment circuit of such a constitution as described above, the analog convergence adjustment signal is controlled by the digital adjustment circuit 5 in a digital process, for the adjustment of convergence according to the digital control signal supplied from the microcomputer 1.
In the conventional hybrid adjustment circuit combining analog and digital circuits, as it is constituted as described above, waveform and level of the analog signal for convergence is adjusted only in the digital adjustment circuit 5. In case the adjustment is carried out only in the digital adjustment circuit 5, there has been a problem that the accuracy of adjustment heavily depends on the temperature characteristic of the digital adjustment circuit 5 and also on the resolution of the digital adjustment circuit 5 itself.